This invention relates to a gas friction pump having an inlet, an outlet and a bell-shaped rotor whose outer surface, together with a first housing part surrounding the rotor, forms an annular, helically extending gas pumping channel. For this purpose, either the outer surface of the rotor or the inner surface of the stator is provided with a helical construction at least in a zone close to the pump outlet. The friction pump further has a second housing part which projects into the space defined by the bell-shaped rotor and which serves for supporting the rotor and the drive motor.
Friction pumps encompass molecular and turbomolecular vacuum pumps. In molecular pumps a movable rotor wall and an immobile stator wall are so configured and so spaced from one another that the pulses imparted by the walls to the gas molecules situated between the walls have a predetermined, preferred direction. For this purpose, as a rule, the rotor and/or stator wall is provided with helically extending (thread or screw-like) depressions or ribs. Turbomolecular pumps have interengaging stator and rotor wheel series, similarly to a turbine; they need a pre-vacuum pressure of approximately 10.sup.-2 mbar. In contrast, molecular pumps deliver at pressures of 10 mbar and above so that the arrangement required for producing the pre-vacuum is much simpler.
Friction pumps of the above-outlined type, such as disclosed, for example, in German Offenlegungsschrift 3,705,912 are frequently used for evacuating vessels in which etching, coating or other vacuum treatments or manufacturing processes are performed. These processes involve the risk that solid particles may gain access to the pumps. In some processes such solid particles may come into being only during the compression of the gases, that is, during the passage of the pumped gas through the vacuum chamber. As an example there is mentioned the formation of aluminum chloride in case of aluminum etching or ammonium chloride in case of coating processes.
In case solid particles of the above-outlined type settle in the gas pumping channels of the vacuum pump, the diameter of the channels is reduced which results in a decrease of the output of the vacuum pump. Precisely in case of friction pumps which are, at least in the outlet-side zone, designed as molecular pumps, it has been found that undesired solid particles settle on the helical channel structure in the vicinity of the pump outlet.
It is a further risk that dust-like solid particles may gain access to the motor chamber which also accommodates bearings. Generally, these bearings are lubricated roller bearings which are exposed to an increased wear when dust is present.
In friction pumps which are utilized in the above-outlined pumping processes, an increased maintenance is necessary for the reasons stated. The removal of dirt from the gas pumping channels and the motor chamber necessitates a disassembly of the pump which is a complex operation, it causes a significant down time and therefore involves substantial expense.